Fixed network communication systems may operate using wire line or radio technology. Wire line technologies include utilizing the utility distribution lines and/or telephone lines. Wireless technologies may utilize the 902–928 MHz range, which can operate without a FCC license through the use of frequency hopping spread spectrum (FHSS) transmission, which spreads the transmitted energy over the band. According to FCC Regulations, for frequency hopping systems operating in the 902–928 MHz band, total output is as follows: 1 watt for systems employing at least 50 hopping channels; and, 0.25 watts for systems employing less than 50 hopping channels, but at least 25 hopping channels. See, 47 U.S.C. § 15.247.
FHSS systems meet the FCC specification by communicating to remote communication devices in synchronization, both in time and frequency. Using this approach, all devices know when to hop to the next channel in the sequence and what the next sequence channel is. A known FHSS system utilizes a hop rate that is faster than the data rate to send multiple sets of randomly selected frequencies in each message to distribute the transmitted energy over the communication band. This distribution is one of the FCC requirements to operate in the ISM band.
A disadvantage of the above is that it requires all devices to include a real time clock, which adds to the cost of the device. In addition, some type of battery storage system is required to maintain the real time clock in the event power should be removed from the device. Further, the requirement to step rapidly through the frequencies constrains the design of such devices and further limits cost reduction.
There have been attempts to utilize remote devices that operate asynchronously that gain synchronization with the transmitter by using a scanning receiver. One example of such a receiver is that of Harmon, U.S. Pat. No. 4,328,581. However, spurs and other unwanted signals interfere with the synchronization process in such receivers. Another problem with such systems is that the receiver is scanning channels based on a list and may be scanning channels other than the channel on which a preamble is being sent. To compensate for this, the transmitter must send the preamble for a period of time long enough to allow the receiver to scan for it and develop a bit timing. In these systems, spurs may be decoded as valid preambles, which adversely affects performance. Further, crosstalk problems may arise if weak signals from the transmitter are captured in the receiver scan sequence prior to determining the desired transmit channel.
Therefore, there is a need for a FHSS communication device that is cost efficient, meets FCC requirements for power distribution in the ISM band, and includes provisions for preventing unwanted signals from capturing the scanning receiver. The present invention is directed to these, as well as other, needs in the art.